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United States Patent O PROCESS OF IMPREGNATING A POROUS ARTICLE Robert D. Mees, Fort Wayne, Ind., assignor to General Electric Company, a corporation of New York Application December 15, 1955, Serial No. 553,400

6 Claims. (Cl. 117-61) This invention relates to processes for treating articles and more particularly to impregnating processes for seaiing, filling and coating articles to obtain desired physical properties. This application is a continuation in part of copending application Serial No. 452,440, led August 26, 1954, for impregnating Process, now abandoned.

In the manufacture of certain articles, such as transformers, particularly in the smaller ratings, a core formed from a stacked plurality of relatively thin laminations of magnetic material is conventionally provided on which one or more coils are arranged. These coils are commonly formed from enameled copper wire with paper insulation arranged between each layer of wire and surrounding the inner and outer peripheries of the coil. ln the design of such transformers, it is important that adequate insulation be provided, not only between the layers of the coils, but also between the coil and the core and the coil and the enclosing case. lt is also important that the transformers have adequate mechanical strength, a low noise level and a low operating temperature. To meet these design objectives, transformers are frequently impregnated with various insulating materials such as asphalt or varnish. This impregnation is intended to till the voids in the coils, particularly in the paper insulation, and to coat the wires, paper and laminations thereby sealing the transformer to prevent moisture absorption and resultant deterioration of the insulation. Impregnation also provides better transfer of the heat generated in the transformer to the surrounding air, renders the unit mechanically rigid thereby preventing physical damage, and reduces the noise produced by the transformer.

In the past, one common method of impregnation involved baking the assembled core and coil unit to remove any moisture entrapped therein, dipping the unit in the impregnating material, and subsequently curing or drying the material. In another method, the core and 'coil unit was after baking placed in a tank. This tank .was then evacuated and the impregnating material ad- -rnitted thereto. Pressure was then applied to the tank after which the units were removed and the impregnant cured. It has been found however, that these impregnation processes did not in every case provide adequa1e impregnation, particularly in the case of transformers having coils formed yof fine tightly wound wire.

It is therefore desirable to provide an impregnating process which will insure complete impregnation of the unit in which all voids are sealed and the unit is completely coated. It is further desirable that such a process be simple, require no more equipment than previous impregnation processes, and lend itself to automatic control of the various steps.

It is therefore an object of this invention to provide an improved impregnating process having the desirable characteristics set forth above.

Further objects and advantages will become apparent by reference to the following description and the accom- ICC 2 panying drawings, and the features of novelty which characterize this invention will' be pointed out with particularity in the claims annexed to and forming a part of this specification.

This invention in its broadest aspects provides an impregnation process in which the article to be impregnated is initially preheated to remove the moisture entrapped therein. An impregnating material having low boiling fractionates is then introduced into the container in which the article is located while the container is maintained substantially at the temperature of the impregnating material. The container is then evacuated with the vacuum being maintained below the point at which the major fractionates of the impregnating material will boil. This controlled evacuation removes the vapors entrapped in the article without creating additional vapors and without removing vapors from the impregnating material. Pressure is then applied to the container to cause the impregnating material to fill the voids in the article. The container is then drained and the impregnating material remaining on the article is hardened or cured depending on the type of impregnating material used.

In the drawing,

Fig. 1 schematically illustrates the equipment used to perform the improved impregnating process of this invention;

Fig. 2 is a flow chart illustrating the steps involved in performing the improved impregnating process of this invention utilizing varnish as the impregnant;

Fig. 3 is a temperature-Vacuum curve for the varnish used in the process of Fig. 2;

Fig. 4 is a liow chart illustrating the steps involved in performing the improved impregnating process of this invention utilizing asphalt as the impregnant;

Pig. 5 is a flow chart illustrating the steps of another impregnating process in accordance with this invention also using asphalt as the impregnant;

Fig. 6 is a temperature-vacuum curve for the asphalt used in the processes of Figs. 4 and 5;

Fig. 7 is a flow chart illustrating the steps of yet another impregnating process in accordance with this invention in which atmospheric pressure is applied after evacuation and varnish is used as the impregnant; and

Fig. 8 is a flow chart illustrating the steps of still another impregnating process in accordance with this invention in which atmospheric pressure is again applied after evacuation and asphalt is used as the impregnant.

Referring now to Figs. l and 2, the process of impregnating the core and coils of small transformers with varnish will be explained as an example of the improved impregnating process of this invention. Here, the core and coil units (not shown) are initially placcd in a preheating oven 1, which has been heated to l5()c C.5" in any suitable manner, as for instance by steam flowing in coil 2. Air is recirculatcd at at least 300 lineal feet pet minute by blower 3 with suitable quantities `of fresh air being introduced through air inlet 4 and an equal quantity exhausted by exhaust fan 5. The units are preheated in oven l in an ambient temperature of 150 CiS" C. until the coldest point therein is at or above the boiling point of water, in this case for 4 hours, in order to remove the moisture entrapped therein. principally in the insulating paper and on the wire of coils; the exhausting of a portion of the oven air by fan 5, and its replacement by fresh air serving to keep the moisture content of the oven air continually below the moisture content of the units to insure transfer of the moisture from the units to the oven air.

After preheating, the units are transferred to impregnating tank 6, which has been heated to C C. in any suitable manner, as by steam flowing in coil 7. Tank 6 is then sealed and the highest vacuum which can be economically obtained, in this case 29 plus inches of mercury, is pulled through vacuum line 8 and valve 9 for 30 minutes, valves 10 and 11 in nitrogen pressure and vent lines 12 and 13 respectively, and valve 14 in drain and varnish line 15 being closed. This evacuation is continued for minutes with tank 6 at 100 C.i5 C. in order to remove air, residual moisture and other vapors entrapped in the units.

Following this, valve 9 is closed to cut otl the vacuum, valve 16 in steam line 17 is closed to cut olf the heating, and valve 18 in water line 19 is opened thereby circulating Water through coil 7 in order to reduce the temperature of tank 6 to substantially Ci-Ce.

the temperature of the incoming varnish. When tank 6 has sufficiently cooled, valves 14, 20 and 21 are opened and the varnish impregnant, which has been cooled to 35 C gg ,is brought into tank 6 from line 22 by pump 23 and the existing vacuum in the tank until the level of the varnish in the tank 6 is approximately 3 inches over the units therein. In this particular example, the varnish used is a thermosetting oxidizing alkyd varnish with a solvent having low boiling constituents.

After the varnish has been introduced into tank 6, valve 14 in varnish and drain line 15 is closed and valve 9 in vacuum line 8 is opened to pull a vacuum over the tliquid varnish in tank 6 for l() minutes. Here, how ever. the vacuum is controlled so that it does not exceed the point at which the low boiling fractionates in the varnish liquid will boil. Referring now to Fig. 3, the curve A is the boiling point curve for the particular varnish used. lt will be recalled that the varnish has been cooled to Assuming that the varnish is at its maximum temperature, i. e., C., the varnish will boil at a vacuum of 24 inches of mercury, on the other hand, if the varnish is at its minimum temperature, i. e., 25 C., the varnish will boil at a vacuum of 28 inches of mercury. Boiling ol the varnish has been found to be highly undesirable since it dissipates the solvents thus changing the characteristics of the varnish and elfecting the impregnation, and the boiling liquids have a tendency to enter and clog vacuum line 8 thus tending to reduce vacuum and consequently overloading the vacuum pump. The initial evacuation of the tank 6 created a vacuum within the core and coil unit. When the relatively cold varnish comes in contact with the still relatively hot units. the low boiling fractionates of the varnish solvent tend to vnporizc and these vapors enter the voids in the unit and tend to prevent the varnish resins from entering and .filling the void. Pulling u vacuum over the liquid varnish in the tank 6 tends to withdraw these entrapped vapors into the varnish permitting the varnish resins to enter the coils in the unit. it is thus seen that it is desirable to pull a vacuum over the liquid varnish in order to remove the vapor entrapped in the units, but that this vacuum should not exceed the point at which the low boiling lractionates of the varnish will boil so that these vapors are not drawn out ol the liquid varnish and into the vacuum line and so that additional vapors are not created. ln order to accomplish this. the vacuum is controlled along curve C with limits between curves B and A, thus, with the varnish at 40 C., the vacuum is maintained at 22 inches of mercury, plus or minus 2 inches, and with the varnish at 25 C., the vacuum is maintained at 26 inches of mercury, plus or minus 2 inches, in any event, below the boiling point of the major fractionates in the liquid varnish, as represented by curve A.

The pulling of a controlled vacuum over the liquid varnish in the tank 6, as explained above tends to draw the vapors entrapped in the core and coil units into the liquid varnish where they are dissolved since the vacuum is not suiciently high to further remove any substantial quantity of vapors from the liquid varnish into the vacuum line 8. Coincident with this withdrawal of vapors from the units, the varnish tends to enter the voids therein. ln order to assist the varnish to enter the voids, valve 9 in vacuum line Sis closed and valve l!) in nitrogen line 12 is opened to apply 90 pounds per square inchtlO pounds per square inch pressure of an inert gas, such as nitrogen over the liquid varnish in tank 6 for l0 minutes.

The first pulling of a controlled vacuum over the liquid varnish in tank 6 may not have removed all of the entrapped vapors since at some point, the forces within the units will have become equalized. Thus, the tirst application of nitrogen pressure over the liquid varnish may not have caused the varnish to enter all the voids since some entrapped vapors remain and here again, the forces within the units will at some point have become equalized. 1n order to insure complete impregnation therefore, the controlled vacuum and nitrogen pressure steps are repeated three additional times thus making a total of four cycles. This repeating of the controlled vacuum and nitrogen pressure steps causes an effect analogous to a pumping action thereby insuring that substantially all of the vapors entrapped in the units are withdrawn to the liquid varnish and that the varnish resins substantially till all of the voids.

After completion of the controlled vacuum and nitrogen pressure steps, valve 10 in nitrogen pressure line 12 is closed, valves 21 and 20 in varnish line 22 are closed, and valves 14, 24, and 25 in drain line 26 are opened. The liquid varnish in tank 6 is then drained therefore using the existing nitrogen pressure over the liquid varnish in the tank and the pump 23. Valves 14, 24, and 25 are then closed, and valves 9 in vacuum line 8 and 16 in steam line 17 are respectively opened. Tank 6 is then heated to 100 C.;t:5 C. while the maximum economically obtainable vacuum is pulled for 60 minutes in order completely to drain excess varnish from the units, to remove solvents, and to speed curing of the varnish. The units are then transferred to oven l and baked for 4 hours in a 150 C. l:5 C. ambient tempera ture with an air ow of at least 300 lineal feet per minute in order to cure the varnish which has impregnated the core and coil units.

It may be further desirable to additionally coat the impregnated core and coil unit with a relatively thin film of varnish and also to ll any voids which may remain after the first impregnation. To accomplish this, the units are again placed in the impregnating tank which has been heated to l00 C5335 C. with valve i6 in steam line 17 open and with valve 9 in vacuum line 8 open. al! other valves being closed. The maximum economically obtainable vacuum is then pulled tor lt) minutes to remove any air and residual vapor entrapped in thc units.

Valve 16 in steam line 17 and valve 9 in vacuum line 8 are then closed cutting off heat and vacuum and valve 18 in water line 19 is opened to cool tank 6 to substantially 35 C.i',,,.c

When the tank is sufficiently cool, valves 14, 20 and 2l are opened and varnish at 35 C-i-flf. is pulled into tank 6 with the existing vacuum therein and with pump 23 to three inches above the units. Valves 14, 20 and 2l are now closed and valve 1t) in nitrogen pressure line 12 is opened and nitrogen at 9() pounds per square inch il() pounds per square inch is applied over the liquid varnish in tank 6 for l0 minutes. Valve 10 is then closed, valves 14, 24 and 25 are opened, and the varnish in tank 6 is drained with the existing nitrogen pressure in tank 6 and pump 23. Valves 18 and 9 are then opened and tank 6 is heated to 100 C.i5 C. while pulling the maximum economically obtainable vacuum for minutes n order to complete draining excess varnish from the units and to remove solvents. The units are then again transferred to oven 1 where they are baked for 4 hours at an ambient temperature of 150 C.15 C. with air-flow of at least 300 lineal feet per minute in order to cure the outer varnish coating on the units.

Referring now to Figs. 1 and 4, the process of impregnating the core and coils of small transformers with asphalt will be explained, as another example of the improved impregnating process of this invention. Here, the core and coil units (not shown) are initially placed in impregnating tank 6, valve 16 in steam line 17 and valve 11 in vent line 13 are opened, and the units are heated at atmospheric pressure for 30 minutes at 150 C.i5 C. Valve 11 is then closed and valve 9 in vacuum line S is opened to pull the maximum economically obtainable vacuum, in this case 29 inches plus of mercury, for 20 minutes with the temperature of tank 6 remaining at 150 (li-5 C. The steps of heating at atmospheric pressure and evacuating are then repeated tive additional times for a total of 6 cycles. This cycling during preheating is provided in order to remove entrapped moisture from the units in the shortest possible time. If vacuum alone is used, an unduly long time is required to insure that all cntrapped moisture is removed. The periodic introduction of air, however, improves the conduction of heat to the units and shortens the overall preheating time.

Following the final preheating evacuating step, valve 9 in vacuum line 8 is closed, valves 14, 24 and 21 are opened and asphalt at 150 C.i5 C. is pulled into the tank 6 to three inches above the units with the existing vacuum therein, the tank temperature remaining at 150 Ci 5 C. In this example, the asphalt used is a petroleum base, 100% solids material with low boiling petroleum derivatives or fractionates.

Referring now additionally to Fig. 6, after the asphalt has been pulled into tank 6, valves 14, 24 and 21 are closed and valve 9 in vacuum line 8 is opened to pull a vacuum over the asphalt in tank 6 for l0 minutes. Here again, the vacuum is controlled so that it does not exceed the point at which the low boiling fractionates of the asphalt will boil. Assuming therefore that the asphalt is at its maximum temperature of 155 C., the low boiling fractionates will boil at 12 inches of mercury and the vacuum is thus maintained at 10 inches of mercury i2 inches of mercury. On the other hand, if the asphalt is at its minimum temperature of 145 C., the low boiling fractionates will boil at 18 inches of mercury and the vacuum is then maintained at 16 inches ofmercury i2 inches of mercury. It is then seen that the vacuum is controlled along curve C between curves B and A and in any event below the boiling point ot the major fractionates as represented by curve A. This pulling of a controlled vacuum over the asphalt in the tank 6 again tends to draw the vapors entrapped in the core and coil units into the liquid asphalt without drawing any substantial quantity of vapors into the vacuum line.

After the step of pulling a controlled vacuum, valve 9 is closed and valve 10 in nitrogen pressure line is opened to apply 90 pounds per square inch nitrogen pressure ll0 pounds per square inch for 10 minutes. The controlled vacuum and nitrogen pressure steps are then repeated four times for a total of five cycles in order to insure that substantially all of the vapors entrapped in the units are withdrawn and that the asphalt has substantially fllled all of the voids.

Following the last nitrogen pressure step, valve 10 is closed to close olf the nitrogen pressure and valves 14, 20 and 25 opened to drain off the excess asphalt from tank 6 with existing nitrogen pressure therein. The impregnated units are then removed from tank 6 and cooled in the open air until the asphalt solidifies.

Referring now to Figs. 1, 5 and 6, another example of an asphalt impregnating process in accordance with this invention will be explained. Here, the core and coils of a small transformer (not shown) are initially placed in oven 1 and preheated 60 minutes in an ambient temperature of 150 C.i5 C. with at least 500 lineal feet per minute air flow. This preheating removes the moisture entrapped in the core and coil units. The units are then placed in tank 6 with the maximum economically obtainable vacuum being pulled for 20 minutes and with the tank heated to 150 C iS" C. The vacuum is then closed off and asphalt at 150 015 C. pulled in with the existing vacuum to three inches above the units. A vacuum controlled to below the point at which the fractionates of the asphalt will boil is then pulled over the asphalt in the tank 6 for 10 minutes, as explained above, and nitrogen pressure of pounds per square inch x10 pounds per square inch is then applied over the asphalt for 10 minutes. These controlled vacuum and nitrogen pressure steps are again repeated four times for a total of ve cycles in order completely to withdraw vapors from the units and to till the voids therein with asphalt. The nitrogen pressure is then shut olf and the excess asphalt in the thank 6 drained with existing nitrogen pressure in the tank. The units are then removed and air cooled to solidify the asphalt.

Referring now to Figs. l and 7, another example of the improved impregnating process of this invention will be explained. It has been found in practice that when smaller impregnating tanks are utilized so that a smaller number of core and coil units are impregnated in one operation, or in the case of larger impregnating tanks when a relatively small number of units are impregnated in one operation, complete impregnation can be obtained by applying atmospheric pressure after the controlled evacuation in contrast with the processes of Figs. 2, 4 and 5 in which a positive pressure above atmospheric is applied. It is thus seen that the pressure differential between the controlled evacuation and pressure steps required to pull the vapor out of the article being impregnated and to force the impregnant into the voids depends on the size of the impregnating tank and the number of articles being impregnated.

In the process of Fig. 7, the core and coil units (not shown) are initially placed in preheating oven 1 and preheated in an ambient temperature of C iS C. with 300 lineal feet per mintue air tlow until the coldest point therein is at or above the boiling point of water. in this case for 45 minutes. The units are then placed in impregnating tank 6, which in this example is unheated, i. e., at room temperature, and the maximum obtainable vacuum is pulled for one minute. The vacuum is then cut off and impregnating varnish at room temperature is pulled into the tank with existing vacuum to one inch above the units. A vacuum is then pulled over the varnish in the tank, the vacuum again being controlled so that it does not exceed the point at which the low boiling fractionates in the varnish will boil, in this case 20 inches of mercury i2 inches, this vacuum being pulled only momentarily. The vacuum is then cut off and vent valve 1l is opened thus applying atmospheric pressure over the varnish in the tank, this application of atmospheric pressure again being momentary. The controlled evacuation and atmospheric pressure steps are then repeated 14 times for a total of 15 cycles after when the excess varnish is drained from the tank and the maximum economically obtainable vacuum pulled for l minute. The impregnated units are then to be removed from tank 6 and if desired, a subsequent overcoat of varnish may be applied in accordance with steps .l through Q of the process of Fig. 2, or by any other suitable process such as by electrostatic deposition.

A modification of the process of Fig. 7 involves the cooling of the units to within C. of room temperature after the initial prcheating and prior to placing them in the imprcgnating tank, i. e., between steps A and B of Fig. 7, all other steps being the same. This modiied process has been found to be particularly desirable where pigmented varnish is used and no subsequent overcoat is applied.

lt will be observed that in thc process of Fig. 7, reduction of the pressure applied to atmospheric has required an increase in the total number of controlled vacuum and pressure cycles.

Referring now to Figs. l and 8, another example of the improved impregnating process of this invention utilizing atmospheric pressure, but with asphalt as the impregnant will be described. Here, the core and coil units are placed in preheating oven l and preheated in an ambient temperature of 165 C.+0 C., 5 C. with 300 lineal feet per minute air ow until the coldest point therein is M5" C., in this case for 45 minutes. This higher prrhcating temperature and higher coldest point temperature is required in this example since an asphalt having a higher inciting point than the asphalt of the processes of Figs. 4 and 5 is used, the 145 C. being slightly higher than the melting point of the asphalt used. The core and coil units arc then placed in impregnating tank 6 which is maintained at a temperature of i" C.:|;5- C. and the maximum economically obtained vacuum is pulled for one minute. The vacuum is then cut ott and vent valve ll opened to reduce the vacuum in thc tank to 28 inches of mercury +0 inch, -2 inches. When this lower vacuum is reached. vent valve 11 is closed and asphalt at 160 C.i5 C. is introduced with existing vacuum to 4 inches above the units. It is to be observed that as the level of the asphalt rises in the tank, the vacuum drops. Vent valve 11 is then opened for 1A: minute thus applying atmospheric pressure over the asphalt in thc tank. The controlled vacuum is then pulled for A minute over the asphalt in the tank, in this case from 22 inches of mercury i2 inches with the asphalt at 165 C. to 26 inches i2 inches with the asphalt at 155 C. The vacuum is then cut oit and vent valve l1 opened for l/ minute thus applying atmospheric pressure over the asphalt in the tank. The controlled evacuation and atmospheric pressure steps are then repeated three times for a total of four cycles after which the excess asphalt is drained from the tank and the units removed.

lt will be readily understood that the above specific imprcgnating processes are by way of example only and that the fundamental pro-cess which involves preheating to remove moisture, introduction of impregnant, the pulling cl a controlled vacuum over the impregnant followed by the application of pressure over the impregnant to withdraw cntrapped vapors without drawing any substantial quantity of vapors into the vacuum line and to assist the impregnant to lill the voids in the article being impregnated, may be used to impregnate other articles in addition to core and coil units for transformers, such as motor coils, paper insulated cable, etc. it will further be understood that other impregnating materials may he used and that variations in the characteristics of the impregnant and in the article being impregnated may in' dic-ate ditlferent specific temperatures, times, and number of repetitions of cycles to those skilled in the art.

lt will non' be readily seen that this improved impregnating process insures more complete impregnation than has heretofore been possible and that it lends itself to the use of automatic equipment for initiating, terminating. and controlling the various steps involved.

While l have illustrated and described specific processer. in accordance with this invention. modifications and improvements therein will occur to those skilled in the art and l intend in the appended claims to cover all -lll u modifications which do not depart from the spirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. The process of impregnating a porous article comprising the steps of: preheating said article in an oven at a temperature of approximately 150 C. for a length of time suicient to raise the temperature of the coldest point in the article to above the boiling point of water; immediately placing said article in an impregnating tank preheated to a temperature of approximately C., scaling said tank and evacuating the same while maintaining the tank at the temperature to which the same was preheated for a length of time suicient to remove air and residual moisture entrapped in said article; cutting ott said vacuum without unsealing said tank and rapidly cooling the same with said article therein to a temperature of approximately 35 C.; immediately introducing a thermosetting varnish having low boiling constituents into said tank with existing vacuum therein until said varnish covers said article, said varnish being at a temperature of approximately 35 C.; immediately sealing Said tank and pulling a vacuum over said varnish therein for a time sufficient to remove vapors entrapped in said article, said last named vacuum being maintained below the point at which the low boiling constituents of said varnish will boil so that substantially no boiling of any part of said varnish occurs; applying pressure to said tank over said varnish for a length of time suieient to cause the same to till the voids in said article; draining excess varnish from said tank; sealing said tank; and immediately heating said tank with said article therein to approximately 100 C. while evacuating the same for a length of time sutlicient to harden the varnish impregnated in and coating said article.

2. The process of impregnating a porous article comprising the steps of: preheating said article in an oven at a temperature of approximately C. with approximately 300 lineal feet per minute air flow thereover for approximately four hours; immediately placing said article in an impregnating tank preheated to a temperature of approximately l00 C., sealing said tank and pulling a vacuum of approximately twenty-nine inches of mercury for approximately thirty minutes while maintaining said tank at the temperature to which the same was preheated; cutting oli said vacuum without unsealing said tank and rapidly cooling the same with said article therein to a temperature of approximately 35 C. plus 5 C. minus 10 C.; immediately introducing a thermosetting varnish having low boiling constituents into said tank with existing vacuum and pumping until the same covers said article, said varnish being at a temperature within the same range as the temperature to which said tank has been cooled; immediately sealing said tank and pulling a vacuum over said varnish therein varying from approximately twenty-two inches of mercury with said varnish at 40 C. to approximately twenty-six inches of mercury with said varnish at 25 C. for approximately ten minutes so that the said low boiling constituents in said varnish do not boil and substantially no boiling of any part of said varnish occurs; applying an inert gas having a pressure of approximately ninety pounds per square inch over said varnish in said tank for approximately ten minutes; closing olf said pressure and draining excess varnish from said tank with existing pressure therein and pumping; sealing said tank; immediately heating said tank to a temperature of approximately l00 C. and pulling a vacuum of approximately twentynine inches of mercury for approximately sixty minutes; and immediately placing the impregnated article in an oven and baking the same therein at a temperature of approximately 150 C. with approximately three hundred feet per minute air How thereover for approximately four hours to completely cure said varnish.

3. The process of impregnating a porous article comprising the steps of: placing said article in an impregnating tank and heating the same at atmospheric pressure at a temperature of approximately 150 C. for a length of time suicient to bring the coldest point in said article to above the boiling point of water; immediately sealing said tank and evacuating the same for a length of time sufficient to remove air and residual moisture entrapped in said article while maintaining the temperature of said tank at approximately 150 C.; cutting ofi said vacuum and immediately introducing a petroleum base 100% solids asphalt having low boiling petroleum derivatives into said tank with existing vacuum until said asphalt covers said article, said asphalt being preheated to a temperature of approximately 150 C. the temperature of said tank being maintained at approximately 150 C.; im mediately sealing said tank and pulling a vacuum over said asphalt therein for a length of time suicient to remove vapors entrapped in said article, said last-named vacuum being maintained below the point at which the low boiling derivatives of said asphalt will boil so that substantially no boiling of any part of said asphalt occurs; applying pressure to said tank over said asphalt for a length of time sufficient to cause said asphalt to lill the voids in said article; draining excess asphalt from said tank; and removing said article from said tank.

4. The process of impregnating a porous article comprising the steps of: preheating said article in an oven at a temperature of approximately 150 C., for a length of time sufficient to bring the coldest point in said article to above the boiling point of water; immediately placing said article in an impregnating tank preheated to a temperature of approximately 150 C., sealing said tank and evacuating the same for a length of time sufcient to remove air and residual moisture entrapped in said article while maintaining said tank at the temperature to which the same was preheated; cutting off said vacuum and immediately introducing a petroleum base 100% solids asphalt having low boiling derivatives into said tank with existing vacuum until said asphalt covers said article, said asphalt being preheated to a temperature of approximately 150 C., the temperature of said tank being maintained at approximately 150 C.; immediately sealing said tank and pulling a vacuum over said asphalt therein for a length of time suicient to remove vapors entrapped in said article, said last named vacuum being maintained below the point at which the low boiling derivatives of said asphalt will boil so that substantially no boiling of any part of said asphalt occurs; applying pressure to said tank over said asphalt for a length of time sufficient to cause said asphalt to fill the voids in said article; draining excess asphalt from said tank; and removing said article from said tank.

5. The process of impregnating a porous article com prising the steps of: preheating said article in an oven at a temperature of approximately 150 C. for a length of time sutlicient to bring the coldest point in said article to above the boiling point of water; immediately placing said article in an impregnating tank which is at a temperature of approximately 35 C., sealing said tank and evacuating the same for a length of time sufficient to remove air and residual moisture entrappcd in said article; cutting off said vacuum and immediately introducing a thermo-setting varnish having low boiling constituents into said tank with existing vacuum until the same covers said article, said varnish being at a ternperature of `approximately 35 C.; immediately scaling said tank and momentarily pulling a vacuum of approximately twenty inches of mercury over the varnish therein. said last named vacuum being below the point at which the low boiling constituents of said varnish will boil so that substantially no boiling of any part of said varnish occurs; immediately and momentarily applying atmospheric pressure to said tank over said varnish; draining excess varnish from said tank; sealing said tank and evacuating the `same for a length of time suflicient to harden the varnish on said article; and removing said article from said tank.

6. The process of impregnating a porous article comprising the steps of: preheating said article in an oven at a temperature of approximately 165 C. for a length of time sulcient to bring the coldest point in said article to above the boiling point of water; immcdiatsly placing said article in an impregnating tank which has been preheated to a temperature of approximately 160 C., sealing said tank and evacuating the same t'or a length of time sufficient to remove air and moisture entrappcd in said article; reducing said vacuum to approximately twenty-eight inches of mercury; cutting oil" said vacuum and immediately introducing a petroleum brise solids asphalt having low boiling derivatives into said tank with existing vacuum until said asphalt covers said article, said asphalt being preheated to a temperature of approximately' 160 C. and said tank being maintained at n temperature of approximately C.: momentarily applying atmospheric pressure over said asphalt in `said tank; sealing said tank and momentarily pulling a vacuum over said asphalt therein. said last named vacuum being maintained below the point at which the low boiling derivatives of said asphalt will boil so that lmbsinntially no boiling of any part of said asphalt occurs: momentarily applying atmospheric pressure over Nid asphalt in said tank; draining excess asphalt from said tank: and removing said article from said tank.

References Cited in the file of this patent UNITED STATES PATENTS 1,729,057 Texier Sept. 24, 1929 2,005,637 Schidrowitz M June 18, i935 2,012,961 Emberg Sept. 3, 1935 2,442,183 Stearns May 25, 1948 2,684,307 Knapman July 20, 1954 FOREIGN PATENTS 461,167 Canada Nov. 15, 1949 

1. THE PROCESS OF IMPREGNATING A POROUS ARTICLE COMPRISING THE STEPS OF: PREHEATING SAID ARTICLE IN AN OVEN AT A TEMPERATURE OF APPROXIMATELY 150*C. FOR A LENGTH OF TIME SUFFICIENT TO RAISE THE TEMPERATURE OF THE COLDEST POINT IN THE ARTICLE TO ABOVE THE BOILING POINT OF WATER; IMMEDIATELY PLACING SAID ARTICLE IN AN IMPREGNATING TANK PREHEATED TO A TEMPERATURE OF APPROXIMATELY 100*C., SEALING SAID TANK AND EVACUATING THE SAME WHILE MAINTAINING THE TANK AT THE TEMPERATURE TO WHICH THE SAME WAS PREHEATED FOR A LENGTH OF TIME SUFFICIENT TO REMOVE AIR AND RESIDUAL MOISTURE ENTRAPPED IN SAID ARTICLE; CUTTING OFF SAID VACUUM WITHOUT UNSEALING SAID TANK AND RAPIDLY COOLING THE SAME WITH SAID ARTICLE THEREIN TO A TEMPERATURE OF APPROXIMATELY 35*C.; IMMEDIATELY INTRODUCING A THERMOSETTING VARNISH HAVING LOW BOILING CONSTITUENTS INTO SAID TANK WITH EXISTING VACUUM THEREIN UNTIL SAID VARNISH COVERS SAID ARTICLE, SAID VARNISH BEING AT A TEMPERATURE OF APPROXIMATELY 35*C.; IMMEDIATELY SEALING SAID TANK AND PULLING A VACUUM OVER SAID VARNISH THEREIN FOR A TIME SUFFICIENT TO REMOVE VAPORS ENTRAPPED IN SAID ARTICLE, SAID LAST NAMED VACUUM BEING MAINTAINED BELOW THE POINT AT WHICH THE LOW BOILING CONSTITUENTS OF SAID VARNISH WILL BOIL SO THAT SUBSTANTIALLY NO BOILING OF ANY PART OF SAID VARNISH OCCURS; APPLYING PRESSURE TO SAID TANK OVER 